It think you can try the static min and static max fields using expressions that take the min/max value of an aggregation.
Min ( Aggr ( Sum ( valuefield ) , DateTimeField ))
Max ( Aggr ( Sum ( valuefield ) , DateTimeField ))
Although it suggests that these are static, when you use an expression then the scaling should change according to what you are selecting, the aggr function should aggregate over a dimension (which shuold be the same dimension as in your chart) and therefore, taking the min/max values should give you what you want. You can of course extend the functionality by rounding these values up/down to the nearest $1M.
this solution is Awesome! I'm still finding that there are peaks in my charts that sometimes poke just above the MAX Y-Axis but apart from that its working much better than anything I've had before.
I get around my problem by adding a nominal amount (or subtracting depending if its a MIN or MAX). ie
Max ( Aggr ( Sum ( valuefield ) , DateTimeField )) + 500
Thank you all the same.